1. Field of the Invention
The present invention relates to a sheet processing apparatus that performs post-processing on a sheet, an image forming apparatus, and a control method.
2. Description of the Related Art
Conventionally, post-processing, such as punching of holes in a sheet (recording sheet) on which an image has been formed by an image forming apparatus is executed by conveying the sheet to a sheet processing apparatus connected to the image forming apparatus. This type of sheet processing apparatus is equipped with a punching mechanism for punching holes in a sheet, and corrects a displacement of the sheet in a sheet width direction orthogonal to a sheet conveying direction (hereinafter referred to as a “lateral displacement”) so as to enhance accuracy of punching positions on the sheet when punching holes in the sheet.
Further, in the sheet processing apparatus, to attain high productivity in which a sheet processing amount is high, there is a case where the amount of lateral displacement of a sheet is detected during conveyance of the sheet whereby the lateral displacement is corrected. As a method of detecting a lateral displacement amount, there has been proposed a method of shifting an optical sensor in the sheet width direction, and detecting the lateral displacement amount based on the time of detection of an edge (lateral edge) of the sheet in the sheet width direction.
When the detection of a lateral displacement amount is executed as described above during conveyance of a sheet, the conveyance amount of the sheet conveyed after the sensor starts to be shifted until it reaches the lateral edge of the sheet is different depending on the lateral displacement amount, and hence the position of detection of the lateral edge of the sheet varies in the sheet conveying direction. This makes it difficult to always detect the lateral displacement amount at a fixed position. Further, when the sheet is skewed, an error can occur between the detected lateral displacement amount and a lateral displacement amount at a trailing edge of the sheet where holes are to be punched. Therefore, to enhance accuracy of punching positions on the sheet, it is required to take a skew feeding rate of the sheet into account when the lateral displacement is corrected.
As a method of detecting a skew feeding rate of a sheet, there has been proposed a method of detecting a lateral displacement and a skew simultaneously in the image forming apparatus (see Japanese Patent Laid-Open Publication No. 2005-342943). In the method disclosed in this publication, a lateral displacement sensor for detecting lateral displacements, disposed at the lateral edge of the sheet, is caused to reciprocate a plurality of times, whereby at least two lateral displacements of the sheet are detected, and a skew feeding rate is detected based on the difference between the results of detection of a plurality of lateral edge positions.
As described above, in the conventional technique, the lateral displacement sensor is caused to reciprocate a plurality of times to thereby detect a plurality of lateral displacements. However, when a sheet conveying speed is increased, there is a fear that after detecting a first lateral edge position of the sheet, the lateral displacement sensor cannot complete detection of second et seq. lateral edge positions of the sheet before the trailing edge of the sheet passes the lateral displacement sensor. To cope with this problem, it is necessary to limit the sheet conveying speed, but if the sheet conveying speed is limited, the productivity of sheet processing can be degraded.
As a method of increasing the productivity of sheet processing, there has been proposed one in which a plurality of lateral displacement amounts are detected by forward and backward operations of the reciprocating operation of the lateral displacement sensor. However, in the lateral displacement sensor, to prevent erroneous detection by noise, threshold voltage of a light receiving circuit is sometimes provided with hysteresis between off to on and on to off (hereinafter referred to as the “directions of detection”) in switching of the lateral displacement sensor. This can cause an error in detection of the lateral edge position due to different directions of detection of the lateral displacement sensor.